As the fibrous carbon material, a carbon nanotube (CNT) and a vapor growth carbon fiber (VGCF) are well known. Both the carbon nanotube and the vapor growth carbon fiber are fine tube form structures constituted with graphene, and are differentiated by the difference in the lamination structure and the fiber diameter associated therewith, as will be described below.
Graphene is a net of honeycomb structure in which six carbon atoms are regularly arranged in a two-dimensional manner, and is also referred to as a carbon hexagonal net plane. The substance in which this graphene is laminated with regularity is referred to as graphite. A single-layer or multiple-layer fine tube form structure constituted with this graphene is a fibrous carbon material, and includes both a carbon nanotube and a vapor growth carbon fiber.
That is, the carbon nanotube is a seamless tube in which graphene is rounded in a tubular shape, and may be a single-layer one or a multiple-layer one in which the layers are concentrically laminated. The single-layer one is referred to also as a single-layer nanotube, and the multiple-layer one is referred to also as a multiple-layer nanotube.
Also, the vapor growth carbon fiber is one having, in a core part, a graphene tube of a single layer or plural layers in which graphene is rounded in a tubular shape, namely a carbon nanotube, where graphite is laminated in a radial direction of the graphene tube so as to surround the core part in a multiple manner and in a polygonal shape, and is referred to also as a super multiple-layer carbon nanotube because of its structure.
In other words, the single-layer or multiple-layer carbon tube that is present at the central part of a vapor growth carbon fiber is a carbon nanotube.
Various composite materials are proposed that aim at improvements in the heat conductivity, electric conductivity, and mechanical properties by the fibrous carbon material while taking advantage of the characteristic features of a metal or ceramics by allowing such a fibrous carbon material to be contained in a metal, ceramics, or further a mixture of these. One of these is a CNT-containing aluminum composite material disclosed in Patent Document 1.
Patent Document 1: International Publication WO2005/040067 pamphlet
This composite material was previously developed by the present inventors, and is one obtained by using a spark plasma sintered body of an aluminum powder as a matrix and mixing a carbon nanotube into this matrix. Aluminum has a high heat conductivity, and is suitable as a matrix of a highly heat-conductive composite material. However, when aluminum is melted in a process of producing a composite material, the aluminum reacts with the carbon nanotube to generate Al—C, thereby considerably deteriorating the material strength. For this reason, it is assumed to be suitable to form aluminum into a matrix by the powder sintering method.
In the powder sintering method, the powder particles are joined with each other by solid phase diffusion at or below the melting point, so that there is no fear that Al—C is generated. However, the produced powder sintered body contains a certain amount of pores, thereby causing a decrease in the heat conductivity. It is the spark plasma sintering method that solves this problem.
The spark plasma sintering method is also referred to as the pulse energization method or the pulse energization pressurizing sintering method, in which, by using a high temperature plasma generated between the powder particles, the adhesion property between adjacent powder particles is enhanced to approximate the porosity within the sintered body infinitely to zero, and also the oxide on the particle surface is made to disappear, thereby contributing to an improvement in the heat conductivity of the matrix itself and an improvement in the heat conductivity between the matrix and the fibrous carbon material.
As to the fibrous carbon material that is allowed to be contained in the matrix as a heat propagation promoting material, it has been found out from recent researches that the heat conductivity will be more improved when a vapor growth carbon fiber having a larger diameter than a carbon nanotube is combined. The vapor growth carbon fiber is easily oriented in a specific direction because of being thicker and longer than the carbon nanotube, so that the effect of improving the heat conductivity in the orientation direction is particularly large.
However, even with a composite material using a spark plasma sintered body of an aluminum powder as a matrix and allowing a vapor growth carbon fiber to be oriented and contained in the matrix, the content of the vapor growth carbon fiber must be made considerably large in order to ensure a high heat conductivity. For example, the heat conductivity of a spark plasma sintered body of an aluminum powder alone serving as a matrix is about 200 W/mK. In order to increase this to about 400 W/mK, which is the double amount, as much as 50 vol % of the vapor growth carbon fiber will be needed.
Since the fibrous carbon material is expensive, the increase in the amount of use thereof directly affects the cost rise of the composite material, so that development of a technique capable of efficiently improving the heat conductivity with a small amount of the fibrous carbon material is waited for.